The present invention relates to a method for producing an organic semiconductor element such as a thin film transistor using an organic semiconductor material.
Organic semiconductor elements having organic semiconductor layers made of organic semiconductor materials are used in devices using logical circuits such as TFTs (thin film transistors), RFIDs (RF tags), or memory used in liquid crystal displays or organic EL displays due to their ability to reduce weight, lower cost, and enhance flexibility.
In the production of an organic semiconductor element, an organic semiconductor layer is typically formed by a vacuum process such as vacuum deposition or a wet process such as a coating method using a coating prepared by dissolving an organic semiconductor material.
On the other hand, a heat transfer (laser heat transfer) method using a laser, such as that described in JP 2007-35742 A or JP 2011-108992 A, is known as a method that does not require a vacuum process or a wet process.
The formation of an organic semiconductor layer by laser heat transfer is performed using a donor substrate having a photothermal conversion layer over the entire surface of a supporting member capable of transmitting a laser beam, and a film (organic semiconductor film) made of an organic semiconductor material over the entire surface of the photothermal conversion layer.
That is, this donor substrate and a substrate to be treated on which an organic semiconductor layer is to be formed (product in which a gate electrode or an insulating layer is formed on a substrate) are laminated so that the organic semiconductor film and the organic semiconductor layer formation surface are opposite one another, and a laser beam is irradiated from the supporting member side of the donor substrate. As a result of the irradiation of this laser beam, the photothermal conversion layer generates heat at the position where the laser beam is incident, and this heat generation causes the organic semiconductor film to heat, melt, and be transferred to the substrate to be treated so that an organic semiconductor layer is formed. Alternatively, by thermal abrasion caused by the heating of the organic semiconductor film, the organic semiconductor film is transferred to the substrate to be treated so that an organic semiconductor layer is formed.
In the formation of an organic semiconductor layer by such laser heat transfer, it is necessary to produce a donor substrate in advance. However, in laser heat transfer, a vacuum process or a wet process is unnecessary for the formation of an organic semiconductor layer on the substrate to be treated, so it is possible to simplify the process of forming the organic semiconductor layer.
In addition, since heat transfer is possible under a wide variety of conditions, it is possible to incorporate treatment for increasing mobility into the process by adjusting the transfer conditions.
Further, laser heat transfer makes it possible to form an organic semiconductor layer having a uniform film thickness with high pattern precision. Therefore, laser heat transfer yields high prospects for being able to produce an organic semiconductor element which realizes high mobility, high pattern precision, and low performance fluctuation at an excellent level.
On the other hand, with laser heat transfer, the photothermal conversion layer, which reaches a high temperature, is also sometimes transferred in addition to the organic semiconductor layer (organic semiconductor material) at the time of transfer. When such a foreign substance is transferred together with the organic semiconductor layer, this causes a reduction in the performance of the organic semiconductor element due to contamination.
The reason that a photothermal conversion layer is necessary is that organic semiconductor materials do not demonstrate absorption with respect to the wavelengths of near-infrared lasers (780 to 1,000 nm) typically used for laser heat transfer.
On the other hand, using a laser beam in the ultraviolet range as a laser beam with which to perform laser heat transfer makes it possible to heat the organic semiconductor material directly, so the photothermal conversion layer can be omitted. However, when a laser beam in the ultraviolet range is used, there is a concern that the organic semiconductor material may be photochemically decomposed or degraded by the laser beam.
On the other hand, a method for producing an organic semiconductor element in which an organic semiconductor layer is formed by heat transfer using a laser beam in the infrared range without using a photothermal conversion layer is described in JP 2004-247716 A.
In this method, an organic semiconductor layer is formed by laser heat transfer in the same manner as described above using a polyimide as a supporting member and using a donor substrate in which an organic semiconductor film is formed on the surface of the supporting member and a carbon dioxide laser with a wavelength of 10.6 μm.
A carbon dioxide laser with a wavelength of 10.6 μm does not transmit polyimides. That is, in this method, a supporting member made of a polyimide is heated by the laser beam instead of a photothermal conversion layer, and an organic semiconductor layer is transferred to the substrate to be treated by this heat.
Therefore, the polyimide, which reaches a high temperature, is transferred together with the organic semiconductor layer, and this may form a contaminant that diminishes the properties of the organic semiconductor element. In addition, polyimides are typically not materials used for the purpose of photothermal conversion. Therefore, with the heating resulting from the heat generation of the polyimide, it may not be possible to sufficiently transfer the organic semiconductor layer to the substrate to be treated depending on the conditions of laser irradiation or the like, and it may not be possible to produce the target organic semiconductor element.